Let resolve_percentages handle flex container box-sizing

Fix #2053.

tests/layout/test_flex.py

@@ -216,6 +216,29 @@ def test_flex_direction_column_reverse_rtl():
     assert div_1.position_y < div_2.position_y < div_3.position_y
 
 
+@assert_no_logs
+def test_flex_direction_column_box_sizing():
+    page, = render_pages('''
+      <style>
+        article {
+          box-sizing: border-box;
+          display: flex;
+          flex-direction: column;
+          height: 10px;
+          padding-top: 5px;
+          width: 10px;
+        }
+      </style>
+      <article></article>
+    ''')
+    html, = page.children
+    body, = html.children
+    article, = body.children
+    assert article.width == 10
+    assert article.height == 5
+    assert article.padding_top == 5
+
+
 @assert_no_logs
 def test_flex_row_wrap():
     page, = render_pages('''

weasyprint/layout/flex.py

@@ -311,15 +311,10 @@ def flex_layout(context, box, bottom_space, skip_stack, containing_block,
     # using the rules of the formatting context in which it
     # participates. For this computation, auto margins on flex items
     # are treated as 0.
-    # TODO: The whole step has to be fixed.
     if axis == 'width':
         block.block_level_width(box, containing_block)
     else:
-        if box.style['height'] != 'auto' and box.style['height'].unit != '%':
-            box.height = box.style['height'].value
-        elif box.style['height'] != 'auto' and containing_block.height != 'auto':
-            box.height = box.style['height'].value / 100 * containing_block.height
-        else:
+        if box.height == 'auto':
             box.height = 0
             for index, child in enumerate(children):
                 if not child.is_flex_item: